The present invention relates to wellhead apparatus forming a slidable fluid-tight seal with a wellhead member, for example, in an oil and/or gas well or a geothermal well.
The wellhead of an oil or gas well serves in part to maintain control over fluid pressure in the well at the surface and to suspend tubing and casing in the well from the surface. It is sometimes desirable to reciprocate the tubing at the wellhead in order to, for example, pull out of or sting into a permanent packer, or to operate downhole tools. At the same time, it is necessary to maintain a fluid-tight seal between the tubing and the wellhead as the tubing is being reciprocated. In the prior art, this was achieved by suspending a joint of production tubing from a wellhead member, such as an adaptor or bonnet positioned above and removably affixed to a tubing head. The tubing head is immovably affixed to the wellhead and has a pack-off device positioned within its bore. The pack-off device forms a slidable seal with the tubing joint permitting it to be reciprocated into and from the well without loss of control over fluid pressure in the annulus between the tubing joint and the casing. A tubing string is suspended from the tubing joint so that it can be reciprocated, for example to pull out of or sting into a permanent packer.
The foregoing arrangement does not permit additional fluid communication lines to be run downhole through the pack-off. Such communication lines are provided, for example, to permit control at the wellhead over various pressure operated downhole tools and instruments, such as pressure operated safety valves and pressure measuring instruments. To achieve such a capability in prior art wellhead arrangements, a tubing head is immovably affixed at the wellhead and a pack-off device is positioned within the bore of the tubing head to form a seal against a reciprocable shroud made up on a coupling which also serves to suspend the tubing within and spaced from the shroud. Accordingly, an annular space is thus defined between the outer surface of the tubing and the inner surface of the shroud. This space serves to accommodate relatively small diameter fluid pressure lines extending from the wellhead downhole, for example, for the operation of a tool, such as a safety joint, or an instrument, such as a pressure measuring device, downhole. The coupling from which the shroud and the tubing are suspended is, in turn, affixed to an adapter having a flange which can be bolted to the tubing head to form a seal thereagainst, and can be unbolted from the tubing head to permit the tubing to be reciprocated upwardly. The adapter also includes ports therethrough which provide communication to the pressure lines through respective fluid conduits formed axially in the body of the coupling intermediate its central bore and its outer surface. The lines extending downhole between the tubing and the shroud suspended from the coupling, are affixed thereto at its lower extremity.
In order to permit the tubing to be reciprocated a sufficient distance, often as much as twenty-four feet, it is necessary for the shroud to provide an outer sealing surface which extends axially in excess of the distance it is desired to reciprocate the tubing. Because this sealing surface must be so long, the prior art shroud is made up of several sections of tubing threaded, welded or soldered together. At each location where the joints are threaded together, leaks can occur. In addition, it is relatively time consuming to make up the tubing to the coupling, the various pressure lines to the coupling, as well as the various sections of shroud to one another and to the coupling, and test each threaded joint for leaks. Where the sections are welded or soldered together they are subjected to stress due to heating which can lead to weld failure and leakage at the wellhead. In addition, because an annular space is defined between the shroud and the tubing, it is necessary to limit the test pressure applied to the outside of the shroud to test the integrity of the seal formed with the packoff and tubing head and the seal between the tubing head and the adapter. Since the wellhead cannot be tested at pressures exceeding the foregoing limit, the usefulness of this arrangement is limited to wells which are not likely to experience greater pressures at the wellhead. A further possibility is that the shroud will burst or the tubing collapse if subjected to high well pressures which sometimes are experienced at the wellhead due, for example, to gas leakage into the tubing/casing annulus.